1. Technical Field of the Invention
The present invention concerns a heat conductive adhesive film requiring a high heat conductivity and a manufacturing method thereof and an electronic component. More particularly, an electrically insulating heat conductive adhesive film capable of dissipating heat generated from components such as semiconductor devices, power source, light source of others used for electric products and an manufacturing method thereof and electronic components excellent in heat radiation.
2. Related Art
Conventionally, various kinds of heat conductive adhesive film have been used for joining heat generating semiconductor devices and heat radiating conductive members, or insulating substrate and metal foil or electrode or the like. In order to increase the heat conductivity, these heat conductive adhesive films have silver, copper, gold, aluminum or other metals and alloys, compounds of high heat conductivity, or aluminum oxide, magnesium oxide, silicon oxide, boron nitride, aluminum nitride, silicon nitride, silicon carbide or other electrically non-conductive ceramics, carbon black, graphite, diamond or other powder particle form or fiber form heat conductive fillers, blended therein.
Among them, electrically non-conductive heat conductive adhesive films filled with boron nitride powder, aluminum oxide powder, aluminum nitride powder or the like, excellent in heat conductivity and electric insulation are largely made fit for practical use.
However, hexagonal boron nitride powder is scaled (thin leaf), and the heat conductivity in its thickness direction (perpendicular to the face) is smaller than the heat conductivity in its plane direction (horizontal to the face) and in case of adhesive film obtained by blending simply boron nitride powder with solid state adhesive and smeared, an adhesive film presenting a sufficient heat conductivity could not be obtained by this simple method, because the plane direction of scaled boron nitride powder is charged into the adhesive film in parallel with the plane direction.
In short, as adhesive films presenting a good electric insulation and a high heat conductivity are not developed, an electrochemical migration was accelerated or corrosion of wiring or pad sections was facilitated by a great amount of heat generated from electronic components such as semiconductor device, composing material cracked or broken, or the interface of the junction of composing material came off due to the generated heat stress, causing various troubles deteriorating the life of electronic components.
Certainly, in the heat conductive adhesive film of Japanese Patent Application No. HEI 11-87482 by the same Applicant as, a diamagnetic filler of 20 W/mxc2x7K or more in heat conductivity was orientated in a given direction in a solid state adhesive, however, the boron nitride powder was not considered as diamagnetic filler.
As the results of diligent and devoted studies in order to solve the aforementioned problems, a manufacturing method of a heat conductive adhesive film taking profit of the characteristics that the heat conductive adhesive film having boron nitride powder orientated in a given direction in a solid state adhesive is excellent in electric insulation and heat conductivity, and that boron nitride powder orientates along the lines of magnetic force in the magnetic field, and an electronic component excellent in heat radiation using the same are to be provided.
Namely, the present invention is a heat conductive adhesive film characterized by that boron nitride powder is orientated in a given direction in a solid state adhesive.
Further, the present invention is a manufacturing method of a heat conductive adhesive film characterized by that a magnetic field is applied to a film composition including boron nitride powder to orientate and solidify the boron nitride powder in the composition in a given direction, and, an electronic component characterized by that a heat generating element and a heat conductive member is adhered with a heat conductive adhesive film having boron nitride powder orientated in a given direction.
Boron nitride powder used in the invention is not specified for the kind of crystalline system, shape and size of powder grain, cohesion degree of powder particle and their distribution. As for the crystalline system, boron nitride powder of hexagonal system, cubic system, wurtzite structure, rhombohedral system, or any other structure can be used. Among them, boron nitride powder of easily available hexagonal system presenting an heat conductivity of some 10 to 100 W/mxc2x7K or so, or of cubic structure presenting an extremely high heat conductivity of 1300 W/mxc2x7K maximum, are preferable.
The particle shape of boron nitride powder is not limited to scaled, or flat shape, but boron nitride powder of various particle forms such as granular, lump, spheric, fiber, whisker shaped boron nitride powder or ground product of them can be used. The particle diameter of boron nitride powder is neither specified; however, the individual average primary diameter in the range of 0.01 to 100 xcexcm, or more preferably, in the range of 0.1 to 20 xcexcm can be used. Finer than 0.01 xcexcm, it becomes difficult to charge in quantity into the heat conductive adhesive film, and boron nitride powder larger than 100 xcexcm is difficult to produce and disadvantageous in respect of the cost. It can not respond, when a thin adhesive layer is required. In case of scaled boron nitride powder, a range of 0.5 to 50 xcexcm as maximum diameter can be easily blended in the film and magnetic field orientated and fit for practical use. Moreover, boron nitride powder of the structure of cohered primary particle is also used.
The concentration of boron nitride powder in the heat conductive adhesive film is preferably 2 to 80 volume % If less than 2 volume %, the improvement effect of heat conductivity is low, and if contained by 80 volume % or more, the viscosity of the composition increases, the fluidity decreases, making the handling difficult, and air bubbles enter inevitably, making impossible to manufacture the desired heat conductive adhesive film, so it is not appropriate. More preferably, the concentration of boron nitride powder in the heat conductive adhesive film is 5 to 50 volume % still more preferably, 10 to 40 volume %. It is also possible to increase the concentration by using boron nitride powders of different power particle diameters at the same time, or surface treatment.
As solid state adhesive used for the invention, material made of publicly known resin or rubber such as epoxy, polyimide, acrylic, vinyl such as polyvinylacetate, urethane, silicone, olefin, polyamide, polyamide-imide, phenol, amino, bismaleimides, polyimide silicone, saturated and non-saturated polyester, diallyl phthalate, urea, melamine, alkyd, benzocyclobutene, synthetic rubber such as polybutadiene or chloroprene, nitrile rubber, natural rubber, styrenic base elastomer or the like that are solid at the room temperature, or become solid in semi-setting state by heating are preferable.
For setting mode, adhesive polymer of any known curing mode, including thermosetting, ultraviolet or visible light setting, room temperature setting, hygrosetting or the like can be used. Among others, at least one kind of heat setting solid state adhesive selected from epoxy, polyimide, acrylic, urethane, silicone based ones that adhere well to various metals, ceramics, plastic material, rubber or elastomer of the material composing the electronic component is preferable.
Further, in case of thermosetting solid state adhesive, a heat conductive adhesive film made to the half set state such as B stage after having blended boron nitride powder and orientated into a given direction, is preferable in respect of adhesion strength and reliability. Also, it is possible to obtain a heat conductive adhesive film presenting an improved wettability with the solid state adhesive or an improved fillability by treating the surface of boron nitride powder with a publicly known coupling agent, in view of surface treatment of the boron nitride powder.
Solvent, thixotropic agent, dispersion agent, hardener, crosslinling accelarator, retarder, adherent, plasticizer, fire retardant, antioxidant, stabilizer, pigment or other known additives can be blended in the heat conductive adhesive film of the invention. Especially, in case when a composition wherein a solid state adhesive and boron nitride powder are blended presents a high viscosity, the magnetic field orientation of the boron nitride powder can be accelerated by lowering the viscosity of the composition by adding a solvent.
Powder form or fiber form metal or ceramics, to be more specific, silver, copper, gold, aluminum oxide, magnesium oxide, silicon nitride, silicon carbide and metal coated resin or other fillers used for the conventional heat conductive film can be used conveniently at the same time. However, it is rather undesirable to mix fillers such as metals presenting a high electric conductance, because an excellent electric insulation is one of characteristics of the heat conductive adhesive film of the invention.
The film thickness is not specified, however, a range of 10 xcexcm to 2 mm is preferable. In case of orientating boron nitride powder to be blended in the thickness direction, it is preferable that the film thickness is thicker than the maximum length of orientation of orientating boron nitride powder to be used, for flattening the heat conductive adhesive film.